Glenn F. Rall, PhD

Our laboratory studies viral infections of the brain and the immune responses to those infections, with the goal of defining how viruses contribute to disease in humans, including cancer. Over the past decades, we have developed mouse models that have enabled mechanistic insights into viral replication and spread within neurons, and the roles played by soluble immune mediators, such as chemokines and cytokines, in viral clearance.

A primary objective of our work is to understand how the immune response contributes to viral clearance from the central nervous system without inducing brain damage, such as encephalitis or edema. Using a transgenic mouse model that restricts measles virus infection to CNS neurons, we have shown that both type I and type II interferons play a crucial role in clearance. Interferons, especially the type I interferons, alpha and beta, are used clinically to dampen virus infections that are associated with human cancer, such as hepatitis C. However, we do not fully understand how these immune molecules function in vivo; thus, a manipulable animal model system is essential for understanding their mode of action, and how they may cause CNS pathology. Moreover, immunotherapy has particular appeal for the resolution of brain tumors that are otherwise not approachable with existing surgical, radiological or chemical strategies. However, immunotherapeutic side effects, such as edema, pose a particular concern in the brain. Therefore, clarifying how a CNS immune response can occur without induction of immunopathology is directly relevant to the development of immune-mediated approaches to resolve CNS tumors.

Type I interferon synthesized by astrocytes limits influenza A virus replication in neurons and prevents egress from the CNS. The mechanisms that govern influenza A virus (IAV) reproduction and pathogenesis in tissues other than the airways and the lungs are not well understood.  This is particularly true in the case of the central nervous system (CNS), even though multiple neurotropic IAV strains can cause severe, even life-threatening, neurological consequences, including encephalitis and meningitis. In this manuscript, an essential role for type I interferons (IFNs) in controlling neurotropic IAV is shown, implicating astrocytes as a primary IFN-producing cell population during IAV CNS infection. Type I IFN receptor 1 knockout (IFNAR KO) mice are significantly more permissive to a neurovirulent strain, A/WSN/1933 (H1N1) (IAV-WSN), than wild type (WT) mice. Moreover, unlike WT mice, in which IAV-WSN remains localized to the CNS, IAV disseminates to other organs in IFNAR KO mice. IFNAR KO mice have negligible leukocyte recruitment into the brain, and leukocyte depletion does not lead to enhanced neuropathology in WT mice, indicating that a CNS-intrinsic immune mechanism drives IFN-mediated viral control during the early phase of infection. Further studies showed that astrocytes are a primary source of intraparenchymal type I IFN: IAV-infected astrocytes produce high levels of type I IFN, and supernatants from primary astrocyte cultures significantly limit IAV replication in neurons (Figure 1). Collectively, these data characterize IAV infection and distribution in the mouse brain, and support the conclusion that astrocyte-derived type I IFN directly controls IAV infection in neurons, leading to restriction of viral reproduction within the brain.

Anti-measles virus antibodies provide long term protection in the infected brain.  Although the contributions of interferons, cytokines, and T cells within the brain following neurotropic virus infection are well-known, how B cells and antibodies contribute to viral control is less well-studied.  Using a mouse model of neuron-restricted measles virus (MV) infection, we have previously shown that wild type mice become infected, but that virus is controlled by T cell-synthesized interferon-gamma.  Recently, we showed that, although IFN-gamma can suppress viral replication in these otherwise healthy mice, viral RNA can persist. When mice are immunosuppressed, viral replication can resume, leading to unique manifestations of central nervous system (CNS) disease. In this manuscript, we show that antibodies, produced by B cells following establishment of a persistent infection, reduce MV RNA levels in vitro and in vivo at timepoints congruent with persistent infection. We conclude that anti-MV antibodies contribute to overall control of MV infection at later stages of infection.

A non-canonical mode of trans-synaptic virus transmission from neurons to astrocytes. Viruses, including herpes-, entero-, and morbilliviruses, are the most common cause of infectious encephalitis in mammals worldwide. During most instances of acute viral encephalitis, neurons are typically the predominant infected cell type. Surprisingly, however, cellular tropism may change during long-term infections in the brain. For example, measles virus (MV), a neurotropic RNA virus, relocates from neurons to astrocytes during viral dormancy in the brains of experimentally infected mice. Consequently, to define how neurotropic viruses trigger neuropathology, it is crucial to define which central nervous system (CNS) cell populations are susceptible and permissive, and to define how viruses spread between these distinct cell populations. Using a MV transgenic mouse model that expresses human CD46 (hCD46), the MV vaccine strain receptor, under the control of a neuron-specific enolase promoter (NSE-hCD46⁺ mice), a novel mode of viral spread between heterotypic cell types of the CNS (specifically, neuron-to-astrocyte) was identified. Although hCD46 is required for initial neuronal infection, this receptor is dispensable for heterotypic spread to astrocytes, which instead depends on glutamate transporters and direct neuron-astrocyte contact. Moreover, in the presence of RNase A, astrocyte infection is reduced, suggesting that the infectious particle that crosses the neuron-astrocyte synaptic cleft is an infectious ribonucleoprotein (RNP)(Figure 2).  The characterization of this unique mode of intercellular transport offers insights into neurotropic viral biology, and may reveal unique targets to mitigate the life-threatening consequences of measles encephalitis.